The use of unsubstituted or substituted imidazole as a curing catalyst in the curing of epoxy resins is known. For example, H. Lee and K. Neville in xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d, pages 10-17 (1967), report the property of epoxy resins cured with 2-ethyl-4-methyl-imidazole.
It has, surprisingly, been found that certain substituted imidazoles in admixture with epoxy resins have an even better latency at room temperature than mixtures of epoxy resins with imidazole or with 2-ethyl-4-imidazole, and at elevated temperature are distinguished by a high reactivity.
The present invention accordingly relates to novel curable mixtures comprising:
a) an epoxy resin having more than one 1,2-epoxy group per molecule and
b) as curing catalyst, an imidazole compound of formula I 
wherein R1, R2 and R3 are each independently of the others a hydrogen atom, a halogen atom, alkyl having from 1 to 20 carbon atoms, alkoxy having from 1 to 20 carbon atoms, unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aralkyl having from 7 to 20 carbon atoms, or unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aryl having from 6 to 20 carbon atoms, and
R4 is alkyl having from 1 to 20 carbon atoms, alkenyl having from 2 to 20 carbon atoms, alkynyl having from 2 to 20 carbon atoms, unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aralkyl having from 7 to 20 carbon atoms or unsubstituted or halo-, nitro, C1-4alkyl- or C1-4alkoxy-substituted aryl having from 6 to 20 carbon atoms. Suitable epoxy resins (a) for the preparation of the curable mixtures according to the invention are the epoxy resins customarily employed in epoxy resin technology. Examples of epoxy resins are:
I) Polyglycidyl and poly(xcex2-methylglycidyl) esters, obtainable by reaction of a compound having at least two carboxy groups per molecule with epichlorohydrin or xcex2-methyl-epichlorohydrin, respectively. The reaction is advantageously carded out in the presence of bases. An aliphatic polycarboxylic acid may be used as compound having at least two carboxy groups per molecule. Examples of such polycarboxylic acids are oxalic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid and dimerised or trimerised linoleic acid. It is also possible, however, to use cycloaliphatic polycarboxylic acids, for example tetrahydrophthalic acid, 4-methyltetrahydrophthalic acid, hexahydrophthalic acid or 4-methylhexahydrophthalic acid. Aromatic polycarboxylic acids may also be used, for example phthalic acid, isophthalic acid or terephthalic acid.
II) Polyglycidyl or poly(xcex2-methylglycidyl) ethers, obtainable by reaction of a compound having at least two free alcoholic hydroxy groups and/or phenolic hydroxy groups with epichlorohydrin or xcex2-methylepichlorohydrin, respectively, under alkaline conditions, or in the presence of an acid catalyst with subsequent alkali treatment.
Such glycidyl ethers are derived, for example, from acyclic alcohols, such as from ethylene glycol, diethylene glycol and higher poly(oxyethylene) glycols, propane-1,2-diol or poly-(oxypropylene) glycols, propane-1,3-diol, butane-1,4-diol, poly(oxytetramethylene) glycols, pentane-1,5-diol, hexane-1,6-diol, hexane-2,4,6-triol, glycerol, 1,1,1-trimethylolpropane, pentaerythritol, sorbitol, and also polyepichlorohydrins. They may also, however, be derived, for example, from cycloaliphatic alcohols, for example 1,4-cyclohexanedimethanol, bis(4-hydroxycyclohexyl)methane or 2,2-bis(4-hydroxycyclohexyl)propane, or they have aromatic nuclei, such as N,N-bis(2-hydroxy-ethyl)aniline or p,pxe2x80x2-bis(2-hydroxyethylamino)diphenylmethane.
The glycidyl ethers may also be derived from mononuclear phenols, for example from resorcinol or hydroquinone, or are based on polynuclear phenols, for example bis(4-hydroxyphenyl)methane, 4,4xe2x80x2-dihydroxybiphenyl, bis(4-hydroxyphenyl)sulfone, 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane, 2,2-bis(4-hydroxyphenyl)propane, 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane, or from novolaks, obtainable by condensation of aldehydes, for example formaldehyde, acetaldehyde, chloral or furfuraldehyde, with phenols, such as phenol, or with phenols that are substituted in the nucleus by chlorine atoms or by C1-C9alkyl groups, for example 4-chlorophenol, 2-methylphenol or 4-tert-butylphenol, or by condensation with bisphenols, for example those of the type mentioned above.
III) Poly(N-glycidyl) compounds, obtainable by dehydrochlorination of the reaction products of epichlorohydrin with amines that contain at least two amine hydrogen atoms. The amines are, for example, aniline, n-butylamine, bis(4-aminophenyl)methane, m-xylylenediamine or bis(4-methylaminophenyl)methane.
The poly(N-glycidyl) compounds, however, also include triglycidyl isocyanurate, N,Nxe2x80x2-di-glycidyl derivatives of cycloalkyleneureas, such as ethyleneurea or 1,3-propyleneurea, and diglycidyl derivatives of hydantoins, such as 5,5-dimethythydantoin.
IV) Poly(S-glycidyl) compounds, for example di-S-glycidyl derivatives that are derived from dithiols, for example ethane-1,2-dithiol or bis(4-mercaptomethylphenyl) ether.
V) Cycloaliphatic epoxy resins, for example bis(2,3-epoxycyclopentyl) ethers, 2,3-epoxycyclopentylglycidyl ether, 1,2-bis(2,3-epoxycyclopentyloxy)ethane or 3,4-epoxycyclohexylmethyl-3xe2x80x2,4xe2x80x2-epoxycyclohexanecarboxylate.
It is also possible, however, to use epoxy resins in which the 1,2-epoxy groups are bonded to different hetero atoms or functional groups; those compounds include, for example, the N,N,O-triglycidyl derivative of 4-aminophenol, glycidyl ethers/glycidyl esters of salicylic acid, N-glycidyl-Nxe2x80x2-(2-glycidyloxypropyl)-5,5-dimethylhydantoin and 2-glycidyloxy-1,3-bis(5,5-dimethyl-1-glycidylhydantoin-3-yl)propane.
For the preparation of the epoxy resin compositions according to the invention it is preferred to use a liquid or solid polyglycidyl ether or ester, especially a liquid or solid diglycidyl ether of bisphenol or a solid or liquid diglycidyl ester of a cycloaliphatic or aromatic dicarboxylic acid, or a cycloaliphatic epoxy resin. It is also possible to use mixtures of epoxy resins.
Suitable solid polyglycidyl ethers and esters are compounds having melting points from above room temperature up to approximately 250xc2x0 C. Preferably, the melting points of the solid compounds are in the range from 50 to 150xc2x0 C. Such solid compounds are known and some of them are available commercially. As solid polyglycidyl ethers and esters it is also possible to use the advancement products obtained by pre-extension of liquid polyglycidyl ethers and esters.
The epoxy resin compositions according to the invention especially comprise a liquid polyglycidyl ether or ester.
The curable mixtures according to the invention comprise as component (b) preferably imidazole compounds of formula I wherein
R1 and R2 are each independently of the other a hydrogen atom, a halogen atom, alkyl having from 1 to 6 carbon atoms, alkoxy having from 1 to 6 carbon atoms or phenyl,
R3 is a hydrogen atom or phenyl, and
R4 is alkyl having from 1 to 10 carbon atoms, alkenyl having from 2 to 10 carbon atoms, unsubstituted or substituted phenyl or unsubstituted or substituted benzyl.
Suitable single or multiple substituents of phenyl or of benzyl are halogen atoms and nitro, C1-C4alkyl- and C1-C4alkoxy groups.
The mixtures according to the invention especially comprise as component (b) an imidazole compound of formula I wherein
each of R1and R3 is a hydrogen atom.
R2 is phenyl, and
R4 is alkenyl having from 2 to 10 carbon atoms, or unsubstituted or substituted phenyl or unsubstituted or substituted benzyl, or wherein
R1is branched alkyl having from 3 to 6 carbon atoms, each of R2 and R3 is a hydrogen atom, and
R4 is unsubstituted or substituted phenyl, unsubstituted or substituted benzyl or branched alkyl having from 3 to 6 carbon atoms.
In an especially preferred embodiment, the mixtures according to the invention comprise an imidazole compound of the formula 
In the curable mixtures according to the invention, the proportion of component (b) is generally from 0.1 to 20% by weight, preferably from 1 to 15% by weight, based on the amount of component (a).
Some of the compounds of formula I are known compounds, which are described, for example, in U.S. Patent Specification U.S. Pat. No. 4,189,543 and can be prepared by reacting
1.) 1 mol of an imidazole compound of the formula 
xe2x80x83wherein R1, R2 and R3 are as defined for formula I, with 1 mol of a chloroformic acid ester of the formula 
wherein R4 is as defined for formula I, or
2.) 1 mol of an imidazole compound of the formula 
xe2x80x83wherein R1, R2 and R3 
are as defined for formula I, with 1 mol of a dicarbonate of the formula 
wherein R4 is as defined for formula I, or
3.) 1 mol of a carbonyldiimidazole compound of the formula 
xe2x80x83wherein R1, R2 and R3 are as defined for formula I, with 1 mol of an alcohol of the formula
R4xe2x80x94OH
wherein R4 is as defined for formula I,
to form compounds of formula I.
Insofar as the compounds of formula I are novel compounds, protection is also claimed for those compounds.
The present invention accordingly relates also to novel imidazole compounds of formula II 
wherein X1, X2 and X3 are each independently of the others a hydrogen atom, a halogen atom, alkyl having from 1 to 20 carbon atoms, alkoxy having from 1 to 20 carbon atoms, unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aralkyl having from 7 to 20 carbon atoms, or unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aryl having from 6 to 20 carbon atoms, and
X4 is alkenyl having from 10 to 20 carbon atoms or alkenyl having from 2 to 20 carbon atoms, or wherein
each of X1 and X3 is a hydrogen atom,
X2 is unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aryl having from 6 to
20 carbon atoms, and
X4 is alkenyl or aryl,
or wherein
X1 is branched alkyl having from 2 to 6 carbon atoms,
each of X2 and X3 is a hydrogen atom, and
X4 is aryl or branched alkyl having from 2 to 6 carbon atoms.
In formula II, preferably
each of X1 and X3 is a hydrogen atom,
X2 is phenyl, and
X4 is alkenyl having from 2 to 6 carbon atoms or unsubstituted or halo-, nitro-, C1-4alkyl- or
C1-4alkoxy-substituted aryl having from 6 to 20 carbon atoms, or
X1 is branched alkyl having from 3 to 6 carbon atoms,
each of X2 and X3 is a hydrogen atom, and
X4 is unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aryl having from 6 to 20 carbon atoms or branched alkyl having from 2 to 6 carbon atoms.
In formula II, especially
X1 is branched alkyl having from 3 to 6 carbon atoms
each of X2 and X3 is a hydrogen atom, and
X4 is unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted phenyl, unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted benzyl or branched alkyl having from 3 to 6 carbon atoms.
Especially preferred are the imidazole compounds of the formulae 
The compounds of formula 11 may also be prepared in accordance with the three above-mentioned processes using the appropriate starting materials.
As mentioned at the outset, the imidazole compounds of formulae I and II are also suitable as curing accelerators in mixtures comprising epoxy resin and curing agent.
The present invention accordingly relates also to curable mixtures comprising
a) an epoxy compound having more than one 1,2-epoxy group per molecule,
b1) as curing accelerator, an imidazole compound of formula I 
xe2x80x83wherein R1, R2 and R3 are each independently of the others a hydrogen atom, a halogen atom, alkyl having from 1 to 20 carbon atoms, alkoxy having from 1 to 20 carbon atoms, unsubstituted or halo, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aralkyl having from 7 to 20 carbon atoms, or unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aryl having from 6 to 20 carbon atoms, and
R4 is alkyl having from 1 to 20 carbon atoms, alkenyl having from 2 to 20 carbon atoms, alkynyl having from 2 to 20 carbon atoms, unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aralkyl having from 7 to 20 carbon atoms or unsubstituted or halo-, nitro-, C1-4alkyl- or C1-4alkoxy-substituted aryl having from 6 to 20 carbon atoms, and
c) a curing agent for epoxy resins.
As curing accelerator it is preferred to use the same imidazole compounds of formula I preferably used as curing catalyst. There is also used as curing accelerator generally from 0.1 to 20% by weight of imidazole compound of formula I, based on the amount of component (a).
The curable mixtures according to the invention may comprise as curing agent (c) a curing agent customarily employed in epoxy resin technology, for example dicyandiamide, polycarboxylic acids and anhydrides thereof, polyamines, polyaminoamides, amino group-containing adducts, aliphatic or aromatic polyols or catalytically active curing agents.
The mixtures according to the invention preferably comprise as curing agent dicyandiamide, a polycarboxylic acid or an anhydride thereof.
There may be mentioned as suitable polycarboxylic acids, for example, aliphatic polycarboxylic acids, for example maleic acid, oxalic acid, succinic acid, nonyl- or dodecyl-succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid and dimerised or trimerised linoleic acid, cycloaliphatic polycarboxylic acids, for example tetrahydrophthalic acid, methylendomethylenetetrahydrophthalic acid, hexachloroendomethylenetetrahydrophthalic acid, 4-methyltetrahydrophthalic acid, hexahydrophthalic acid and 4-methylhexahydrophthalic acid, and aromatic polycarboxylic acids, for example phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, pyromellitic acid and benzophenone-3,3xe2x80x2,4,4xe2x80x2-tetracarboxylic acid, and the anhydrides of the mentioned polycarboxylic acids.
The amount of curing agent used depends upon the chemical nature of the curing agent and upon the desired properties of the curable mixture and of the cured product. The maximum amount can be determined readily. When the curing agent is an amine, usually from 0.75 to 1.25 equivalents of amine hydrogen are used per equivalent of epoxide. When polycarboxylic acids or anhydrides thereof are used, usually from 0.4 to 1.1 equivalents of carboxy group or anhydride group, respectively, are used per equivalent of epoxide. When polyphenols are used as curing agent, from 0.75 to 1.25 phenolic hydroxy groups are used per equivalent of epoxide. Catalytically active curing agents are generally used in amounts of from 1 to 40 parts by weight per 100 parts by weight of epoxy resin.
The curable mixtures according to the invention may also comprise the fillers and reinforcing materials customarily employed in epoxy resin technology. Suitable fillers include, for example: mineral and fibrous fillers, such as quartz powder, fused silica, aluminium oxide, glass powder, mica, kaolin, dolomite, graphite, carbon black, and also carbon fibres and textile fibres. Preferred fillers are quartz powder, fused silica, aluminium oxide and dolomite. Suitable reinforcing materials are, for example, glass fibres or carbon fibres.
The curable mixtures according to the invention are prepared according to methods known per se, for example using known mixing apparatus, for example stirrers, kneaders, rollers or, in the case of solid substances, dry mixers.
The curing of the curable mixtures according to the invention to form mouldings, coatings or the like is carried out in a manner customarily employed in epoxy resin technology, as described, for example, in xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d, 1967, by H. Lee and K.Neville.
The curable mixtures according to the invention are excellently suitable as casting resins, laminating resins, adhesives, compression moulding compounds, foamed materials, coating compounds and also as encasing systems for electrical and electronic components, especially as laminating resins or foamed materials.
The present invention accordingly relates also to the moulded materials, coatings or bonded materials manufactured from the curable mixtures according to the invention.